Humans are excellent at selecting the relevant part of a cluttered visual scene or the relevant conversation at a noisy party. In contrast, humans are often not so successful at dividing attention over multiple stimuli. One cannot read two books at once and is it is not wise to talk on a phone and drive at the same time. Much has been learned about the effects of attention on physiological responses in the human and monkey visual cortex. However nearly all of this work has addressed selective attention, which is when attention is directed to one source of information over another. In general, studies of selective attention have shown that activity in many areas of the brain is greater for a stimulus that is relevant to the current task compared to a stimulus that is not relevant. Surprisingly, very littl is known about the effects of divided attention - paying attention to more than one thing on a time - on neuronal responses. This lack of a physiological literature is particularly surprising given the long history of research on the effects of divided attention on behavioral performance. Interestingly. these behavioral studies show a wide range of effects: for discrimination of simple features there can little cost to attended to multiple stimuli at a time, whereas for higher-level perceptual tasks such as reading words it may impossible to attend to more than one stimulus at a time. Here we propose a series of behavioral and imaging studies to examine the physiological basis of divided attention. We will (1) examine what factors in a task result in a cost when dividing attention. In particular we will examine whether it is the complexity of the stimulus or the task that is the critical factor for both a simple grating task (Specific Aim 1) an complex lexical task (Specific Aim 2). Second we will determine the cause of reduced neural responses and impaired behavioral performance when attentional capacity is limited. In particular, we will determine whether attentional limitations are due to attenuation of attentional gain, a shift to serial processing or suppressive interactions between stimuli. Finally we will examine the spatial profile of attentional modulations during divided attention: whether it is spread broadly across space and/or features or allocated discretely. This gap in the literature is of clinical importance. Individuals with autism spectrum disorder and ADHD show differential divided attention effects: a deeper understanding of the mechanism underlying divided attention is likely to prove critical in linking these behavioral differences to underlying neurophysiologica mechanisms.